The first-generation commercial high-temperature superconductors utilize two members of homologous series Bi2Sr2CanCu(n+1)O6+2n+x with n=3, abbreviated 2223, and n=2, abbreviated 2212. Herein x stands for the appropriate oxygen content at which the respective compound exhibits superconductivity.
Besides the desired superconducting phases 2223 and 2212 other phases can be found within the superconductor material such as 2201, copper oxides, oxides of alkaline earth metals etcetera. Further, within the grains of 2223 and 2212, intergrowths of other members of homologous series can be found, e.g. short sequences of 2201 and 2223 atomic layers in the 2212 grains.
However, for good superconductor performance in the final superconductor the content of such second phases and intergrowths should be as small as possible and, further, well connected and aligned grains of the superconductor material are required.
In particular 2212 superconductors are widely used since the 2212 phase is more stable than the 2213 phase and can be, in general, obtained by a simpler processing technique. Further, 2212 superconductors can be fabricated in more diverse shapes such as tapes and wires with a great variation of cross sections, for instance of round, square or rectangular geometry, in form of Ag-sheathed monocore and multifilamentary tapes and wires, thick films on various substrates, bulk plates etcetera.
It has been shown, that the final microstructure and superconductor performance strongly depend on the properties of the precursor material used for preparing the superconductor.
In general, as the precursor material a powder mixture of appropriate compounds of the constituent elements with the atomic ratio of the desired superconductor material is used, that is in case of Bi2212 of Bi:Sr:Ca:Cu of 2:2:1:2. Techniques for preparing the powder mixtures are generally known, e.g. by chemical mixing the constituent elements in solutions in form of nitrates and their subsequent decomposition via, e.g. aerosol spray pyrolysis, oxalate coprecipitation followed by spray-drying, freeze-drying etcetera.
It is known that the properties of the superconductor material can be improved if a precursor powder is used having high homogeneity of composition and small particle size.
For fabricating the superconductors the so called oxide-powder-in-tube (OPIT) technique can be advantageously used. In this technique, the precursor powder is inserted into a tube of normal conducting material, commonly Ag or an Ag-alloy are used, so called billets. The filled billets are deformed, e.g. by drawing, to produce monofilamentary wires.
These monofilamentary wires are usually assembled in new billets of Ag or Ag-alloy, that are deformed, e.g. by drawing and/or rolling, into multifilamentary wires and tapes, respectively. This process can be repeated several times leading to superconductor geometries of different complexity. A typical final filament diameter in the resulting multifilamentary wires is 10 to 20 μm.
The resulting Bi2212/Ag superconductors are then subjected to a heat treatment, the so called partial melt processing (PMP). The PMP of Bi2212 superconductors aims to obtain well connected, phase-pure and well aligned Bi2212 material. During PMP the superconductor is heated above the decomposition temperature of the 2212 phase in order to eliminate porosity and random grain orientation and then is slowly cooled down to crystallize new 2212 grains. This inverse peritectic reaction of 2212 formation is difficult to complete because of problems resulting from consuming second phases grown during the melting step. Optimal processing is usually considered to be a compromise between good connectivity which requires longer time in the melt state and phase purity which requires dissolution of the second phases which grow while in the partial melt state.
Commonly used precursor material has high homogeneity in composition and usually a particle size of 1 to 5 μm. Further, the Bi2212 grains of the commonly used precursor material show 10 to 20% intergrowths of 2201.
There was a need for further optimisation of the partial melt processing. In particular, there was a need for obtaining a final superconductor material having good connectivity between the grains on the one hand, but, on the other hand, without or with a minimum formation only of second phases during the melt processing.